Fix null-conditional field-like event raise NRE with no subscribers

[DavidObando]

Summary

A null-conditional raise Evt?(args) of a field-like event threw NullReferenceException when the event had no subscribers instead of being a no-op. This broke the canonical Evt?(...) / Evt?.Invoke(...) short-circuit for both nominal EventHandler and structural (object?, EventArgs) -> void events, and blocked 3 migrated Oahu tests at Oahu.Core.SettingsBase.OnChange.

Root cause (regression of #2792)

PR #2792 (fix/2726) canonicalizes conventional structural event shapes and nominal EventHandler events to the CLR System.EventHandler / EventHandler<T> delegates. Their invocation therefore now routes through the CLR-delegate call path in OverloadResolver.BindCallExpression (dispatch via Invoke). Unlike the FunctionTypeSymbol and named-delegate paths — which already guard the void raise via BuildIndirectDelegateCall — that path emitted an unconditional callvirt Invoke, dereferencing a null backing delegate on a fresh event.

Commit bisect (from the issue): structural evt_struct_null raise works at 2621fa0a (#2791) and breaks at c84ca771 (#2792); both nominal and structural fail at the cycle-16 tip.

Fix

In the CLR-delegate call path, when a null-conditional invocation targets a void-returning CLR delegate, bind Invoke against a captured receiver and wrap it in a BoundNullConditionalAccessExpression, so a null backing delegate short-circuits to a safe no-op — matching C# Evt?.Invoke(args). This covers both nominal and structural field-like events with no Oahu-specific logic. Value-returning conditional invocations keep their existing lowering.

Emitted IL for the raise now becomes the standard null guard:

ldfld <event>; stloc.0; ldloc.0; brtrue not_null; br end; not_null: ...callvirt Invoke; end:

Tests

New Issue2796NullConditionalEventRaiseEmitTests (runtime + ILVerify via the shared IlVerifier.Verify helper):

• nominal EventHandler zero-subscriber raise no-ops; with a subscriber invokes
• structural (object?, EventArgs) -> void zero-subscriber raise no-ops; with a subscriber invokes
• unsubscribe-then-raise no-ops (the SettingsBase.OnChange failure mode)
• generic EventHandler<T> zero-subscriber no-op and subscribed invoke

Validation

• Compiler.Tests: 3495 passed, 0 failed
• Core.Tests: 6704 passed, 1 skipped
• Minimal repros (evt_raise.gs, evt_struct_null.gs no-op & exit 0; evt_nonnull.gs hits=1) pass and ILVerify clean
• Migrated Oahu.Core (recompiled with the patched gsc): the 3 previously-failing tests pass; full migrated Oahu.Cli.Tests 342/342 pass (was 339/342)
• All suites run with MSBUILDDISABLENODEREUSE=1

Fixes #2796